Quantitatively deciphering the relationship between muscle activation and thickness deformation is essential for diagnosing muscle-related diseases and monitoring muscle health (e.g., Facioscapulohumeral Dystrophy). Despite the potential of ultrasound (US) imaging and sensing to measure changes in muscle thickness during movements, it remains challenging to make a fully portable device, considering the wiring and data collection. On the other hand, surface electromyography (sEMG) can record muscle bioelectrical signals and measure muscle activations, offering a unique perspective that correlates with underlying changes in muscle thickness. This paper introduces a deep-learning-based approach that used sEMG signals to infer muscle deformation. Using a hierarchical combination of self-attention and cross-attention mechanisms, this method predicted muscle deformation directly from sEMG data, eliminating the dependency on applying ultrasound imaging techniques. The experimental results on six healthy subjects indicated that our approach could accurately predict muscle excursion with an average precision of 0.923 $\pm$ 0.900 mm, showing benefits in measuring muscle deformation only with a sEMG device. This technique facilitates real-time portable muscle health monitoring by sEMG to provide bioelectrical signals and biomechanical information. It indicates the great potential of using this technique in clinical diagnostics, sports science, and rehabilitation.

In Total Knee Replacement Arthroplasty (TKA), surgical robotics can provide image-guided navigation to fit implants with high precision. Its tracking approach highly relies on inserting bone pins into the bones tracked by the optical tracking system. This is normally done by invasive, radiative manners (implantable markers and CT scans), which introduce unnecessary trauma and prolong the preparation time for patients. To tackle this issue, ultrasound-based bone tracking could offer an alternative. In this study, we proposed a novel deep-learning structure to improve the accuracy of bone tracking by an A-mode ultrasound (US). We first obtained a set of ultrasound dataset from the cadaver experiment, where the ground truth locations of bones were calculated using bone pins. These data were used to train the proposed CasAtt-UNet to predict bone location automatically and robustly. The ground truth bone locations and those locations of US were recorded simultaneously. Therefore, we could label bone peaks in the raw US signals. As a result, our method achieved sub-millimeter precision across all eight bone areas with the only exception of one channel in the ankle. This method enables the robust measurement of lower extremity bone positions from 1D raw ultrasound signals. It shows great potential to apply A-mode ultrasound in orthopedic surgery from safe, convenient, and efficient perspectives.

In breast cancer patients, some nodules are only visible on MRI, thus, requiring MRI-guidance to perform the biopsy. MRI interventions are cumbersome due to the magnetic field and the constrained working space. An MR safe robotic system actuated by pneumatic stepper motors may enable these procedures, improving both accuracy and image-guided navigation. A compact multipurpose pneumatic stepper motor has been designed with outer dimensions$(45 \times 40\times 15)\mathbf{mm}^{\mathbf{3}}$. This is configurable as a linear, rotational or curved stepper motor with a customizable step size and radius of curvature. Five copies of these motors actuate the Sunram 7 biopsy robot, of which the moving part (without protruding racks and tubes) measures$(130 \times 65\times 55)\mathbf{mm}^{\mathbf{3}}$. After manually choosing the target location and angle of approach, the needle is robotically inserted into the breast and the integrated pneumatic biopsy gun is fired to sample tissue from the lesion. The maximum torque of the presented motor is 0.61 N m at 6 bar which can be achieved using 13-teeth polycarbonate gears. Using 17-teeth gears for higher accuracy and a more convenient working pressure of 2 bar the maximum torque is 0.28 N m. The accuracy in free air of the Sunram 7 robot is 1.69mm and 1.72mm in X and Z-direction respectively, with a resulting 2-D error of 2.54 mm. The workspace volume is 4.1 L. When targeting 10 mm-sized lesions in phantoms under MRI guidance, Sunram 7 achieved a success rate of 68%. The minimum interval between two successive biopsies was 5:47 minutes. The presented multipurpose stepper motor has distinct advantages over previous designs in terms of robustness, customizability, printability and ease of integration in MR safe robotics. The Sunram 7 is able to perform accurate MRI-guided biopsies in a large workspace volume while reducing the intervention time when compared to the gold standard (i.e., MRI-guided free-hand biopsy).

Absolute position detection in sensorless electric stepper motors potentially allows for higher space efficiency, improved shock resistance, simplified installation, reduced number of parts and lowered cost. A prototype is demonstrated measuring 42 × 42 × 34 mm3with seven coils arranged in a star configuration. The rotor is ϕ 25.8 × 12.5 mm2and has 51 teeth which are irregularly spaced. At the driver side, the coil currents are measured during motion in order to reconstruct the absolute position of the motor. Calibration and smoothing techniques are used to reduce systematic and stochastic measurement errors, respectively. The motor is able to detect and correct its position after externally-induced stalls at the tested motor speeds from 40 rpm to 108 rpm. The holding torque is 0.23 N m at an armature current of 1 A; on average the torque is 7% lower than that of a reference bipolar stepper motor with the same dimensions. The results show that dynamic position sensing and correction are possible for a range of velocities, but not at standstill. The driver requires seven current sensors and sufficient computational power, and proper calibration of motor intrinsics is required beforehand. The presented technology could make existing 3-D printers and other machines with open-loop stepper motors more robust and increase the range of operating speeds and accelerations, without the adverse side-effects of increased complexity and cost associated with dedicated position sensors.

Humans are capable of managing multiple tasks simultaneously. It is widely assumed that human motor control can be emulated by impedance control. To achieve human-like behavior, however, the impedance parameters of multiple tasks may vary during task execution. We propose an algorithm that shapes task impedance as a function of the robot’s time-varying inertial properties. These properties involve virtually constrained masses and virtually constrained inertias that counteract a task in order to comply with a given constraint. In this work, we not only detect task conflicts, but also show how to handle them. Our method is able to control kinematically redundant robots. We developed a damping-design method that does not interfere with our desired Cartesian task-space behavior. The control approach was verified in experiments on a real robot. We compared our impedance shaping method with two alternative control approaches: simple impedance superposition and nullspace projection. Our method preserved the passivity while improving the Cartesian task performance of an impedance controller. The method has computational advantages, beneficial to control robots with many degrees of freedom.

This work addresses the interaction control problem of a fully actuated aerial vehicle considered as a flying end-effector. We tackle the problem using geometrically consistent variable-stiffness impedance control for safe wrench regulation using the concept of energy tanks, where both the modeling and the control are carried out in the port Hamiltonian framework. We exploit previous well-known results in the literature of ground manipulators and extend them to be applied for novel and challenging aerial physical interaction with a focus on quasi-static applications. The energy-awareness of the presented control method guarantees the stability of the aerial robot in both free-flight and in-contact scenarios together with a level of safety in the case of contact-loss with the unknown environment. Furthermore, by utilizing bond graphs we demonstrate how the closed-loop passivity can be graphically conducted. The validity of our proposed approach is shown via several experiments. We also provide several insights on how the proposed framework could be extended to a generic dynamic aerial physical interaction.

Breast cancer affects one out of eight women. Ultrasound (US) plays an important role in the diagnostic workflow, especially during the biopsy phase, in which tissue is extracted from the lesion for further analysis. The extension from 2D to 3D US acquisitions has multiple benefits including enhanced lesion localization and improved registration with MRI data. Current commercial 3D US systems lack the ability to preserve the breast’s original shape. Robotic US scanners follow tailored trajectories and produce high quality volumes by accurate localization of 2D slices captured with a conventional linear probe. Current methods require a patient specific model to plan the scanning trajectory.In this study we investigate how to change the direction of the scanning trajectory based on US feedback, such that no patient specific model is required to perform a scan. In our method, the scanning trajectory is kept tangent to the breast based on confidence maps of the US images and an estimation of current radius of curvature of the surface. We evaluated our approach on a realistic breast phantom. The robot revolves around the breast without prior knowledge of its shape. In ten scans, the RMS error between the probe’s scanning plane and the breast’s surface normal is 12.6° out-of-plane, and 4.3° in-plane. A 3D US reconstruction shows the acquired data. This is a step forward to fully autonomous, high quality robotic US volume acquisitions.

We address the problem of motion control for safe physical interaction, and in particular finding new ways for impedance controller parameters’ adaptation to ensure better safety with minimum possible lose in robot performance. We propose an exteroception-based dynamically updated safety metric that takes into account current robot state and inertia as well as external objects’ mass, shape, material properties, velocity, sensor confidence and existing sampling rates. We also present how this metric can be applied to design a finite state machine of the multi-stage controller, which allows us to prioritize either safety of performance by setting different energy and power constraints with smooth transition in between free motion and interaction modes.

We present an optimization-based structural-parametric synthesis method for reconfigurable closed-chain underactuated linkages for robotic systems that physically interact with the environment with an emphasis on adaptive grasping. The key idea is to implement morphological computation concepts to keep both necessary trajectory-specific holonomic constraints and mechanism adaptivity using variable length links (VLL), while we evolve from a fully actuated to an underactuated system satisfying imposed design requirements. It allows to minimize the number of actuators, weight, and cost but keep high payload and endurance that are not reachable by tendon-driven designs. Despite the method is general enough, for clarity, we demonstrate its use on a number of finger mechanisms for adaptive grippers.

One way to create a quadruped galloping robot from scratch is to design a brick-shaped body and utilize relatively simple open-chain leg mechanisms controlled with relatively complex control algorithms. Alternatively, we can look at how nature solved the same task designing fast mammals such as cheetah, and by means of morphological computation, we can design a complex mechanical system that has much of the desired behavior within inherent dynamics and only a little control effort is needed to stabilize or augment the motion.In this paper, we have analyzed a real cheetah motion using video tracking and looked for a way to match the dynamic model parameters of the real cheetah with a galloping robot with an elastic spine. We believe the elastic spine is the essential feature for a fast-running energy-efficient galloping robot. Within this paper, we are focused on the flying stage when the elastic spine affects the motion of the robot’s front and rear bodies. We have found how to optimize mass distribution and elasticity in the spine in order to get the cheetah-like galloping motion of a quadruped robot.

Reinforcement learning algorithms have proven to be capable of solving complicated robotics tasks in an end-to-end fashion without any need for hand-crafted features or policies. Especially in the context of robotics, in which the cost of real-world data is usually extremely high, Reinforcement Learning solutions achieving high sample efficiency are needed. In this paper, we propose a framework combining the learning of a low-dimensional state representation, from high-dimensional observations coming from the robot’s raw sensory readings, with the learning of the optimal policy, given the learned state representation. We evaluate our framework in the context of mobile robot navigation in the case of continuous state and action spaces. Moreover, we study the problem of transferring what learned in the simulated virtual environment to the real robot without further retraining using real-world data in the presence of visual and depth distractors, such as lighting changes and moving obstacles. A video of our experiments can be found at: https://youtu.be/rUdGPKr2Wuo.

Breast cancer is one of the most diagnosed types of cancer worldwide. Volumetric ultrasound breast imaging, combined with MRI can improve lesion detection rate, reduce examination time, and improve lesion diagnosis. However, to our knowledge, there are no 3D US breast imaging systems available that facilitate 3D US - MRI image fusion. In this paper, a novel Automated Cone-based Breast Ultrasound System (ACBUS) is introduced. The system facilitates volumetric ultrasound acquisition of the breast in a prone position without deforming it by the US transducer. Quality of ACBUS images for reconstructions at different voxel sizes (0.25 and 0.50 mm isotropic) was compared to quality of the Automated Breast Volumetric Scanner (ABVS) (Siemens Ultrasound, Issaquah, WA, USA) in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and resolution using a custom made phantom. The ACBUS image data were registered to MRI image data utilizing surface matching and the registration accuracy was quantified using an internal marker. The technology was also evaluated in vivo. The phantom-based quantitative analysis demonstrated that ACBUS can deliver volumetric breast images with an image quality similar to the images delivered by a currently commercially available Siemens ABVS. We demonstrate on the phantom and in vivo that ACBUS enables adequate MRI-3D US fusion. To our conclusion, ACBUS might be a suitable candidate for a second-look breast US exam, patient follow-up, and US guided biopsy planning.

Reinforcement Learning has been able to solve many complicated robotics tasks without any need for feature engineering in an end-to-end fashion. However, learning the optimal policy directly from the sensory inputs, i.e the observations, often requires processing and storage of a huge amount of data. In the context of robotics, the cost of data from real robotics hardware is usually very high, thus solutions that achieve high sample-efficiency are needed. We propose a method that aims at learning a mapping from the observations into a lower-dimensional state space. This mapping is learned with unsupervised learning using loss functions shaped to incorporate prior knowledge of the environment and the task. Using the samples from the state space, the optimal policy is quickly and efficiently learned. We test the method on several mobile robot navigation tasks in a simulation environment and also on a real robot. A video of our experiments can be found at: https://youtu.be/dgWxmfSv95U.

This work presents the concept of energy-based safety for series-elastic actuation. Generic actuation passivity and safety is treated, defining several energy storage and power flow properties related to passivity. Safe behaviour is not guaranteed by passivity, but can be guaranteed by energy and power limits that adapt the nominal behaviour of an impedance controller. A discussion on power flows in series-elastic actuation is presented and an appropriate controller is developed. Experimental results validate the effectiveness of the energy-based safety in elastic actuation.

Current challenges in automated robotic breast ultrasound (US) acquisitions include keeping acoustic coupling between the breast and the US probe, minimizing tissue deformations and safety. In this paper, we present how an autonomous 3D breast US acquisition can be performed utilizing a 7DOF robot equipped with a linear US transducer. Robotic 3D breast US acquisitions would increase the diagnostic value of the modality since they allow patient specific scans and have a high reproducibility, accuracy and efficiency. Additionally, 3D US acquisitions allow more flexibility in examining the breast and simplify registration with preoperative images like MRI. To overcome the current challenges, the robot follows a reference- based trajectory adjusted by a visual servoing algorithm. The reference trajectory is a patient specific trajectory coming from e.g. an MRI. The visual servoing algorithm commands in-plane rotations and corrects the probe contact based on confidence maps. A safety aware, intrinsically passive framework is utilised to actuate the robot. The approach is illustrated with experiments on a phantom, which show that the robot only needs minor pre-procedural information to consistently image the phantom while relying mainly on US feedback.

To this day, most robots are installed behind safety fences, separated from the human. New use-case scenarios demand for collaborative robots, e.g. to assist the human with physically challenging tasks. These robots are mainly installed in work-environments with limited space, e.g. existing production lines. This brings certain challenges for the control of such robots. The presented work addresses a few of these challenges, namely: stable and safe behaviour in contact scenarios; avoidance of restricted workspace areas; prevention of joint limits in automatic mode and manual guidance. The control approach in this paper extents an Energy-aware Impedance controller by repulsive potential fields in order to comply with Cartesian and joint constraints. The presented controller was verified for a KUKA LBR iiwa 7 R800 in simulation as well as on the real robot.

The integration of computer vision techniques for the accomplishment of autonomous interaction tasks represents a challenging research direction in the context of aerial robotics. In this paper, we consider the problem of contact-based inspection of a textured target of unknown geometry and pose. Exploiting state of the art techniques in computer graphics, tuned and improved for the task at hand, we designed a framework for the projection of a desired trajectory for the robot end-effector on a generically-shaped surface to be inspected. Combining these results with previous work on energy-based interaction control, we are laying the basis of what we call vision-based impedance control paradigm. To demonstrate the feasibility and the effectiveness of our methodology, we present the results of both realistic ROS/Gazebo simulations and preliminary experiments with a fully-actuated hexarotor interacting with heterogeneous curved surfaces whose geometric description is not available a priori, provided that enough visual features on the target are naturally or artificially available to allow the integration of localization and mapping algorithms.

Modern robots act in dynamic and partially unknown environments where path replanning can be mandatory if changes in the environment are observed. Task-prioritized control strategies are well known and effective solutions to ensure local adaptation of robot behaviour. The highest priority in a stack of tasks is typically given to the management of correct robot operation or safe interaction with the environment such as obstacles or joint limits avoidance, that we can consider as constraints. If a constraint makes impossible achieving a certain task, such as tracking a Cartesian trajectory, a local control algorithm partially sacrifices the latter which is only accomplished to the best of the robot's ability to generate internal motions. In this control framework, problems may occur in some applications, like in the surgical domain, where it is not safe that some tasks are simply sacrificed without prior notice. The contribution of this work is to introduce a coordinate invariant index, that is used to provide a geometrical interpretation of task conflicts in a task-priority control framework and to develop a method for on-line detection of algorithmic singularities, with the goal of increasing safety and performances during robot operations.

One out of eight women will get breast cancer during their lifetime. A biopsy, a procedure in which a tissue sample is acquired from the lesion, is required to confirm the diagnosis. A biopsy is preferably executed under ultrasound (US) guidance because it is simple, fast, and cheap, gives real-time image feedback and causes little patient discomfort. However, Magnetic Resonance (MR)-detected lesions may be barely or not visible on US and difficult to find due to deformations of the breast. This paper presents a robotic setup and workflow that assists the radiologist in targeting MR-detected breast lesions under US guidance, taking into account deformations and giving the radiologist robotic accuracy. The setup consists of a seven degree-of-freedom robotic serial manipulator equipped with an end-effector carrying a US transducer and a three degree-of-freedom actuated needle guide. During probe positioning, the US probe is positioned on the patient's skin while the system tracks skin contact and tissue deformation. During the intervention phase, the radiologist inserts the needle through the actuated guide. During insertion, the tissue deformation is tracked and the needle path is adjusted accordingly. The workflow is demonstrated on a breast phantom. It is shown that lesions with a radius down to 2.9 mm can be targeted. While MRI is becoming more important in breast cancer detection, the presented robot-assisted approach helps the radiologist to effectively and accurately confirm the diagnosis utilizing the preferred US-guided method.

Passivity is a necessary condition for a system's stability, meaning that an energy generating system may readily become unstable. Energy-aware actuation can enforce passivity by monitoring the amount of energy that is exchanged with a system, while using an allocated energy budget to execute a task. Careful communication of the energy budgets is important to prevent accidental generation of energy. Therefore, this paper proposes an energy transaction protocol to communicate energy budgets in a distributed robotic system to guarantee that passivity is kept. Simulations are performed with a model of the protocol that is applied to a simulated unreliable communication channel. It is verified that the proposed protocol keeps passivity in the system, while a naive communication strategy either violates passivity or is unnecessarily dissipative.

Despite the recent advances in soft endoscopes, they could not yet fully fulfill the requirements for minimally invasive and natural orifice transluminal endoscopic surgeries. Maneuverability, bendability, different structural stiffness required for different endoscopic surgical interventions, the space needed for surgical manipulators and patient’s safety are amongst the main factors which can contribute to implementing the new soft robotics endoscope in practice. In this study, based on finite element analysis on an existing endoscopic segment, a new improved endoscopic module was developed. A novel approach for stiffening of the endoscopic module was proposed. The actuation and stiffening components were combined to introduce a multi-level stiffening mechanism to the endoscope, and also to provide a free lumen for manipulators. To increase patient’s safety, a force sensing module was developed to estimate the magnitude and direction of the force from tissues to the endoscope. The developed endoscopic system was integrated to a haptic control system. The 3D kinematics control and haptic feedback control of the endoscopic module were validated.

In this work, we show how the interactive behavior of an aerial robot can be modeled and controlled effectively and elegantly in the port-Hamiltonian framework. We present an observer-based wrench/impedance controller for a fully-actuated hexarotor. The analysis and control are performed in a geometrically consistent manner on the configuration manifold of the special Euclidean group SE (3) such that the UAV's nonlinear geometric structure is exploited. The controller uses a wrench observer to estimate the interaction wrench without the use of a force/torque sensor. Moreover, the concept of energy tanks is used to guarantee the system's overall contact stability to arbitrary passive environments. The reliability and robustness of the proposed approach is validated through simulation and experiment.

Cardiovascular diseases remain as the most common cause of death worldwide. Remotely manipulated robotic systems are utilized to perform minimally invasive endovascular interventions. The main benefits of this methodology include reduced recovery time, improvement of clinical skills and procedural facilitation. Currently, robotic assistance, precision, and stability of instrument manipulation are compensated by the lack of haptic feedback and an excessive amount of radiation to the patient. This paper proposes a novel master-slave robotic platform that aims to bring the haptic feedback benefit on the master side, providing an intuitive user interface, and clinical familiar workflow. The slave robot is capable of manipulating conventional catheters and guidewires in multi-modal imaging environments. The system has been initially tested in a phantom cannulation study under fluoroscopic guidance, evaluating its reliability and procedural protocol. As the slave robot has been entirely produced by additive manufacturing and using pneumatic actuation, MR compatibility is enabled and was evaluated in a preliminary study. Results of both studies strongly support the applicability of the robot in different imaging environments and prospective clinical translation.

The biomimetic approach in robotics is promising: nature has found many good solutions through millions of years of evolution. However, creating a design that enables fast and energy-efficient locomotion remains a major challenge. This paper focuses on the development of a full leg mechanism for a fast and energy-efficient 4-legged robot inspired by a cheetah morphology. In particular, we analyze how the allocation of flexible elements and their stiffness affects the cost of transport and peak power characteristics for vertical jumps and a galloping motion. The study includes the femur and full leg mechanism's locomotory behavior simulation, capturing its interaction with the ground.

Pneumatic rotational stepper motors can be used to actuate MR (magnetic resonance) safe robotic systems. This paper describes novel techniques to minimize the volumetric size and/or step size of such motors in order to cope with the limited space requirements while still delivering high precision. Three designs are presented: the R-10 measures 1.0 cm3, has step size 12.9° and torque 1.2 N mm. The R-40 measures 25.6 cm3, has step size 1.01° and torque 470 N mm. The R-54 measures 46.7 cm3, has step size 1.01 m° and torque 240 N mm. The particularly small step size in the R-54 motor is achieved by using a high-reduction planetary gear.These three motors demonstrate that small-scale rotational stepper motors with a wide range of step sizes and good torque characteristics can be constructed that surpass state-of-art designs by a considerable margin. This allows the advancement of MR safe robotics towards more compact and versatile designs, and also overcome certain existing limitations by combining multiple motors with different specifications.

This paper presents the autonomous battery exchange operation for small scale UAVs, using a mobile ground base that carries a robotic arm and a service station containing the battery exchange mechanism. The goal of this work is to demonstrate the means to increase the autonomy and persistence of robotic systems without requiring human intervention. The design and control of the system and its components are presented in detail, as well as the collaborative software framework used to plan and execute complex missions. Finally, the results of autonomous outdoor experiments are presented, in which the ground rover successfully localizes, retrieves, services, and deploys the landed UAV, proving its capacity to extend and enhance autonomous operations.

Safety is essential for robots in unknown environments, especially when there is physical Human-Robot Interaction (pHRI). Control over energy, or passivity, is an effective safety mechanism. However, when the control algorithm is implemented in a discrete-time computer, computation and communication delays readily lead to loss of passivity and to instability. In this paper, a way to make the actuators aware of the energy that they inject into the system is presented. Passivity and stability are then always guaranteed, even in situations of total communication loss. These Embedded Energy-Aware Actuators are a model-free passivity and safety layer that make complex robotic systems dependable, well-behaved and safe. The proposed method is validated in simulation and experiments.

Breast cancer is the most frequently life-threatening diagnosed type of cancer among women. Early and accurate diagnosis by acquiring a tissue sample using biopsy techniques is essential. However, small lesions only visible by MRI are often missed in standard methods, indicating the need for a robotic-assisted biopsy system that is MRI-compatible. Existing proof-of-concepts are difficult to employ due to large sizes and/or actuation complexities. Therefore, a compact pneumatically-actuated 5 DOF MRI-compatible robot was further developed and controlled by a computerized valve manifold. Accuracy and efficiency measurements have been performed using two different PVC breast phantoms with embedded fish oil capsules (mimicking lesions) inside a 0.25T MRI scanner. Preliminary results show that the end-effector was able to hit the targeted capsules, and that the position accuracy is in the range of 4.7-7.3 mm. The developed robotic system has potential to perform MRI-guided breast biopsies accurately and improve the clinical workflow.

In the field of aerial robotics, one of the key challenges is to enable aerial manipulators to exert substantial forces on the environment. Enabling this will allow the technology to perform meaningful tasks airborne, such as cleaning or grinding surfaces. While in contact and applying a large, continuous force, control of the UAV's attitude is a challenge. In this work, we show that a regular (PID-based) attitude controller is incapable of stabilizing aerial manipulators that apply physical contact forces on the environment that are comparable to the UAV's weight. We present a novel control algorithm that uses an LQR-optimized state feedback on the roll and yaw angle while in contact. Experiments on a UAV of 1.5 kg show that the proposed controller is capable of applying a contact force of over 15 N - equal to the UAV's weight - sustained for several minutes.

Targeting of small lesions with high precision is essential in an early phase of breast cancer for diagnosis and accurate follow up, and subsequently determines prognosis. Current techniques to diagnose breast cancer are suboptimal, and there is a need for a small, MRI-compatible robotic system able to target lesions with high precision and direct feedback of MRI. Therefore, the design and working mechanism of the new Stormram 4, an MRI-compatible needle manipulator with four degrees of freedom, will be presented to take biopsies of small lesions in the MRI scanner. Its dimensions (excluding racks and needle) are 72×51×40 mm, and the system is driven by two linear and two curved pneumatic stepper motors. The T-26 linear motor measures 26×21×16 mm, has a nominal step size of 0.25 mm and the measured maximum force is 63N at 0.65 MPa. The workspace has a total volume of 2.2 L. Accuracy measurements have shown that the mean positioning error is 0.7 mm, with a reproducibility of 0.1 mm. Velocity measurements with 5 m long tubes show a maximum stepping frequency of 8 Hz (maximum force) to 30 Hz (unloaded). These results show that the robot might be able to target lesions with sub-millimeter accuracy within reasonable time for the MRI-guided breast biopsy procedure.

This paper proposes a modeling method for generic compliant robotic manipulators. It is based on graph theory and the port-Hamiltonian formalism, which allows a modular approach to the interconnection of rigid bodies with compliant actuators by means of kinematic pairs. This modularity enables a simple and straight-forward adaption the model when a manipulator's actuator morphology is changed. An example of a spatial three degree-of-freedom manipulator shows that this modeling method is more suitable for modeling changes in actuator placement than the traditional Euler-Lagrange method.

The paper focuses on the mechatronic design of a robotic manipulator that is meant to be mounted on an Unmanned Aerial Vehicle (UAV) and to be used in industrial applications, for both aerial inspection by contact and aerial manipulation. The combination of an UAV and the robotic manipulator realizes the aerial manipulator. The robotic manipulator is designed to be versatile so that the aerial manipulator can perform both trajectory tracking in free flight and physical interaction. Moreover, the robotic manipulator can be mounted on commercially available UAVs a modular way without interfering with the existing onboard control architecture. Experimental test are validating the overall mechatronic design.

Variable stiffness actuators (VSA) are fostered in robotics for their capability to address physical interaction with a physically adjustable compliance, being advantageous in terms of efficiency, safety and adaptability to unknown environments. Here we introduce the concept of a switching VSA (sVSA), in which a single actuator is used to control the position or the stiffness of a robotic joint according to a mechanical switch. Despite not allowing simultaneous control of both quantities, this architecture has the potential to make the design lighter, requiring one continuously powered actuator, controllable in position, and one additional switch, activated only occasionally between two limit stages: the advantages are the separation of the motors power requirements and a simpler control. A first prototype of a 1-DoF revolute variable-stiffness joint has been built, based on the vsaUT-II developed at the University of Twente, with a novel clutch mechanism allowing continuous and efficient switching. The prototype proved functionality and feasibility of the sVSA concept.

Quadrupedal running is an efficient form of locomotion found in nature, which serves as an inspiration for robotics. We believe that a resonance-based approach is the path towards energy-efficient legged locomotion and running robots. The first step in working towards this goal is creating an energy-efficient one-legged hopper. Such a one-legged hopper was designed and constructed. The impact efficiency of the mechanism is calculated analytically, determined in simulation and measured with the prototype. The impact efficiency as calculated from the experiments is found to be in agreement with the analytical expectation and simulation results. Finally, using an electric motor to inject energy by creating a virtual spring with reverse hysteresis, hopping is achieved.

This paper proposes a modeling method for generic serial-chain compliant robotic manipulators. It is based on graph theory and port-Hamiltonian systems, which allows a modular approach to the interconnection of rigid bodies with compliant actuators by means of kinematic pairs. This modularity allows a very simple and straight-forward change in a manipulator's actuator morphology. An example of a two degree of freedom planar manipulator shows that this modeling method is more suitable for modeling changes in actuator placement than traditional Euler-Lagrange models.

This paper proposes a semi-autonomous bilateral control architecture for unmanned aerial vehicles. During autonomous navigation, a human operator is allowed to assist the autonomous controller of the vehicle by actively changing its navigation parameters to assist it in critical situations, such as navigating through narrow paths. The overall goal of the controller is to combine the stability and precision of an autonomous control with the cognitive abilities of a human operator, only when strictly required for the accomplishment of a task. The control architecture has been validated through simulations and experiments.

This paper discusses the design of a vehicle for in-pipe inspection using omnidirectional wheels and active stabilizing control. A novel propulsion mechanism is discussed using omnidirectional wheels (or omni-wheels) is presented which allows direct control of the orientation in the pipe. This paper will show the development and evaluation of a prototype model. Rapid prototyping techniques have been used in this proof-of-principle.

Synchronisation is an essential part of many controlled dynamical systems, in particular in the limb motion of legged robots. In this paper we introduce a novel control strategy that allows synchronisation of two oscillators without using any external power, but by modulating the power flow between the two oscillators. We then derive a separate energy-level controller that regulates the oscillation amplitude, again without changing the system's total energy. Finally, we show that the strategy works on a realistic mechanical system, by synchronising the phase difference and apex height of two bouncing masses.

In robotics, differential mechanisms are widely used when lightweightness and compactness are a requisite for the robot design. Moreover, the last decades have seen the rise of (variable) compliant acuators as important elements to perform safe interaction and dynamic tasks. This paper introduces a Variable Stiffness Differential Drive (VSDD), i.e., a differential transmissions with variable stiffness actuators (VSAs), and presents a dynamic analysis. The analysis shows that, when variable stiffness actuators are used in coupled differential transmissions, the allowable stiffness range at the output of the system depends on the position of the springs inside the device. In particular, independent output joint stiffness can be obtained by dislocating the actuation from the elastic elements.

This paper presents a new free-flight controller for aerial manipulators, unmanned aerial vehicles endowed with a robotic manipulator. The control strategy exploits the dynamics of the manipulator to improve the tracking performance and maneuverability of the UAV by expanding its flight envelop. The controller increases the spectrum of deployable manipulators, which otherwise are limited to be light-weighted manipulators and with a dynamics that do not significantly affect the UAV. The effectiveness and applicability of the proposed controller is verified through simulations and experiments.

This paper presents a general passivity based interaction controller design approach that utilizes a combined energy and power based safety norms to assert safety of domestic robots. Since these robots are expected to co-habit the same environment with a human user, analysing and ensuring their safety is an important requirement. Safety analysis of domestic robots determine whether a robot achieves a desired safety level according to some quantitative safety metrics. When it comes to controller design for human friendly robots, it often involves introducing compliance and ensuring asymptotic stability using impedance control technique and passivity theories. The controller proposed in this work also uses a passive design that extends the standard impedance control scheme with energy and power based safety metrics to ensure that safety requirements defined in these norms are achieved by domestic robots. The effectiveness of the proposed guideline is illustrated with simulation and experimental results.

A rail-guided robotic system is currently being designed for the inspection of ballast water tanks in ships. This robotic system will manipulate sensors toward the interior walls of the tank. In this paper, the influence of rail compliance on the end-effector position error due to ship movement is investigated.

In this paper, a modular method of modeling compliant robotic systems using graph theory is treated. Graph theoretic analyses ensure a structured way of describing a system and allow a straightforward extension to more complex systems. The graph models of a series elastic actuator, a variable stiffness actuator and a multi degrees of freedom compliant system are derived. These systems are controlled using an optimal control law that is able to find the optimal stiffness setting and distribution to accomplish a certain task. A case study shows a multi degrees of freedom compliant system which is required to resonate at the output and to accomplish a back-and-forth motion. It is shown that a constant optimal stiffness is found in the resonance simulation, and a varying optimal stiffness in case of the back-and-forth task. This indicates that this methodology can assist in finding an optimal stiffness distribution of complex robotic systems for a given task.

This paper presents a versatile control architecture for aerial robots in interactive tasks. The control architecture is characterized by its unique capability of varying the apparent impedance of the controlled aerial robot as well as the interaction force, when in contact. This work finds its way in various applications where different impedance and interaction force controllers provide high task performances as well as safety. The feasibility and effectiveness of the proposed controller are demonstrated by experimental results preformed on a quadrotor aerial robot.

In this study, we present the implementation of the controller for adapting the energy storage capacity of the WalkMECH according to the different walking speeds and gait characteristics of an amputee. Since the main aim is to keep the design both mechanically and metabolically energy-efficient, the actuation system is designed based on the minimal actuation principle. The overall system, called WalkMECHadapt, is evaluated with the experimental test set-up with a healthy subject. Test results show that the system is working sufficiently for adapting the energy storage capacity of the WalkMECHadapt thanks to the simple nature of the controller architecture.

This paper presents a generic hierarchical passive teleoperation control architecture that effectively addresses the issues of workspace incompatibility and precision, as well as other classical and peculiar challenges. More specifically, the control scheme consists of a user-defined variable scale mapping, a variable impedance master controller, and a virtual slave system. The port-based modeling framework has been extensively used in our formulation, providing more insight about energetic flows in the system that are particularly useful for the design of a passive controlled system. Moreover, various practical considerations that are required for the effective usage of the control architecture are discussed. The achieved better precision and overall task performance have been validated and verified by elaborate simulations and experiments.

The bipedal spring-loaded inverted pendulum (SLIP) model captures characteristic properties of human locomotion, and it is therefore often used to study human-like walking. The extended variable spring-loaded inverted pendulum (V-SLIP) model provides a control input for gait stabilization and shows robust and energy-efficient walking patterns. This work presents a control strategy that maps the conceptual V-SLIP model on a realistic model of a bipedal robot. This walker implements the variable leg compliance by means of variable stiffness actuators in the knees. The proposed controller consists of multiple levels, each level controlling the robot at a different level of abstraction. This allows the controller to control a simple dynamic structure at the top level and control the specific degrees of freedom of the robot at a lower level. The proposed controller is validated by both numeric simulations and preliminary experimental tests.

In this paper, we present the design, simulation and experimental validation of a control architecture for an unmanned aerial vehicle endowed with a manipulation system and interacting with a remote environment. The goal of this work is to show that the interaction control allows the manipulator to track a desired force, normal to a vertical wall, while still maintaining the possibility of moving on the wall. The control strategy has been implemented and validated in simulations and experiments on the manipulator standalone, i.e., attached to a fixed base, and on the manipulator attached to the aerial vehicle.

In this work, we propose a hybrid model for a bipedal walker with controlled variable leg stiffness, and a control strategy for stable gait control. The control reference is a passive gait of the limit-case bipedal spring-loaded inverted pendulum model with massless feet, ensuring that the gait is close to the ideal passive gait and thus aiming for energy efficiency. The effectiveness of the controller is demonstrated with numerical simulation results. From the results a theoretical cost of transport is calculated, showing that the control strategy is indeed energy efficient.

This paper describes the theory and practice for a stable haptic teleoperation of a flying vehicle. It extends passivity-based control framework for haptic teleoperation of aerial vehicles in the longest intercontinental setting that presents great challenges. The practicality of the control architecture has been shown in maneuvering and obstacle-avoidance tasks over the internet with the presence of significant time-varying delays and packet losses. Experimental results are presented for teleoperation of a slave quadrotor in Australia from a master station in the Netherlands. The results show that the remote operator is able to safely maneuver the flying vehicle through a structure using haptic feedback of the state of the slave and the perceived obstacles.

In this paper two different approaches to guarantee stability of bilateral telemanipulation systems are discussed. Both approaches inject damping into the system to guarantee passivity of the interaction with the device in the presence of time delays in the communication channel. The first approach derives tuning rules for a fixed viscous damper, whereas the second approach employs modulated dampers based upon the measured energy exchange with the device and enforces passivity in the time domain. Furthermore, a theoretical minimum damping injection scheme is sketched that shows that the fixed damping approach is inherently conservative with respect to guaranteeing stability. Experimental results show that both the theoretical minimum damping scheme and a time domain passivity algorithm are successful in stabilizing the telemanipulation system for large time delays with lower gains of the damping elements than derived by the fixed damping injection approach. However, as damping is inherently present in the system, the fixed damping tuning rules can be used to identify if a time domain passivity algorithm is needed given boundary conditions on the actual time delays.

In this paper, the vsaUT-II, a novel rotational variable stiffness actuator, is presented. As the other designs in this class of actuation systems, the vsaUT-II is characterized by the property that the output stiffness can be changed independently of the output position. It consists of two internal elastic elements and two internal actuated degrees of freedom. The mechanical design of the vsaUT-II is such that the apparent output stiffness can be varied by changing the transmission ratio between the elastic elements and the output. This kinematic structure guarantees that the output stiffness can be changed without changing the potential energy stored internally in the elastic elements. This property is validated in simulations with the port-based model of the system and in experiments, through a proper control law design, on the prototype.

In this paper, we present the mechanical design and modeling of a manipulation system for unmanned aerial vehicles, which have to physically interact with environments and perform ultrasonic non-destructive testing experiments and other versatile tasks at unreachable locations for humans. The innovation of the prototype lies in the use of a three degrees of freedom Delta robotic manipulator together with a nondestructive testing end-effector, realized by a Cardan gimbal that allows the ultrasonic sensor to compliantly interact with the remote environment. The Cardan gimbal is endowed with a small actuator for the roll motion of the end-effector, a compliant element in the direction of interaction and two passive rotational degrees of freedom with defined equilibria to overcome gravity and to define a stable zero reference. Simulation results of a ducted-fan unmanned aerial vehicle interacting with a wall validate the overall mechanical design.

In this paper, we present haptic teleoperation of underactuated unmanned aerial vehicles by providing a multidimensional generalization of the virtual slave concept. The proposed control architecture is composed of high-level and low-level controllers. The high-level controller commands the vehicle to accomplish specific tasks and renders both the state and the environment of the vehicle to the operator through haptic feedback. The low-level controller interprets the command signals from the operator, regulates the dynamics of the vehicle and feeds back its state to the high-level loop. Passivity of the teleoperation loop is always ensured independently of the choice of implementation of the low-level controller and the configuration of the flying hardware by a passivity-enforcing supervisor, which associates every action of the slave with an energy expense that can only be made available from a multi-state energy tank. The effectiveness of the proposed algorithm is illustrated with simulations and experimental tests.

A system is developed that can reconstruct the pose of flexible endoscopic instruments that are used in advanced flexible endoscopes using solely the endoscopic images. Four markers are placed on the instrument, whose positions are measured in the image. These measurements are compared to a three-dimensional rendered model of the instrument. The pseudo-inverse of the interaction matrix between the state of the model and the marker positions in the image is used to update the state such that the model will track the real instrument. An experiment was performed in which the instrument was moved inside a colon model, while the tip position was simultaneously measured with an electromagnetic tracking system. The root mean square errors of the position estimation were 2.3 mm, 2.2 mm and 1.7 mm in the horizontal (x), vertical (y) and away-from-camera (z) directions, respectively.

This video shows a system that estimates the pose of a flexible endoscopic instrument, based on the endoscopic images. A three-dimensional rendering of the instrument is matched to the actual instrument that is observed through the endoscopic camera. This system was evaluated in an anatomical model of a colon. The estimated position of the tip of the instrument was compared to measurements performed with an electromagnetic tracker. The errors of the position estimation were 2 mm, 2.2 mm and 1.7 mm in the horizontal (x), vertical (y) and away-from-camera (z) directions, respectively.

In this paper, we present a haptic teleoperation control algorithm for unmanned aerial vehicles, applying a kinetic scrolling-based position mapping. The proposed algorithm overcomes the master workspace limitations and enables to teleoperate the aerial vehicle in unbounded workspace in a fast and intuitive manner. Moreover, it provides high precision to teleoperation tasks. Simulation and experimental results validating the applicability and effectiveness of the proposed algorithm are also presented.

This paper proposes a new impedance control principle for mechanical systems which have driving systems whose control input is not the torque any more. It is shown how to transform such mechanical systems with driving systems into another systems with the desired mechanical impedance. Also it is proved that the proposed principle achieves a robustness in the presence of parameter perturbations. First, we propose a new state space expression of mechanical systems with driving systems based on a natural Casimir function. Second, we propose an impedance control using an artificial Casimir function as well as the natural Casimir function. Third, we analyze a robustness not on the closed-loop stability but on the closed-loop structure. Finally, the validity of the proposed method is confirmed by numerical simulations. Remarkably, the closed-loop system keeps the mechanical impedance structure robustly and thus achieves a robust behavior.

Legged locomotion involves periodic negative and positive work, which usually results in high power consumption. Improvement of the energy efficiency is possible by using energy storage elements to reversibly store the negative work performed during a walking or running cycle. While series elastics with high impedance (high gear ratio) actuators are widely used, we investigate the application of parallel stiffness with highly backdriveable actuators. We specifically show that the use of parallel springs in a bounding quadruped with a flexible spine can lower power consumption by over 50%.

This paper focuses on the modeling and control of a flying robot. The complete system, composed of a quadrotor unmanned aerial vehicle and a custom-made manipulator, has been designed for remote inspection by contact of industrial plants. The goal of this paper is to show the dynamical characteristics of the flying robot during tasks that require physical interaction, and to determine a control strategy that allows to safely interact with unknown environments. The methodology has been implemented on a real prototype and tested in an indoor area. Experimental results validate the proposed controller and show its effectiveness.

This video presents practical realizations and comparison between three different haptic tele-control algorithms of aerial vehicles. These strategies, besides addressing the classical issues of stability and transparency, provide different alternatives for overcoming challenges that are peculiar to haptic teleoperation of aerial vehicles. The experimental results show the performance and effectiveness of the proposed control algorithms even in the presence of significant time delays.

This paper deals with the bilateral teleoperation of underactuated aerial robots by means of a haptic interface. In particular, we propose a switching-based state mapping and control algorithm between a rate-based passive controller, which addresses the workspace incompatibility between the master and slave systems, and a pose-based passive controller, which is required for precise operation. The overall control architecture provides the possibility of changing the scaling factor of the mapping online, while preserving the passivity of the complete system. In our formulation, we use the port-Hamiltonian framework, in which energetic considerations play a determinant role for passivity and, thereby stability of the overall system. Simulation and experimental results illustrating the effectiveness of the proposed algorithm are also presented.

Most of today's robots have rigid structures and actuators requiring complex software control algorithms and sophisticated sensor systems in order to behave in a compliant and safe way adapted to contact with unknown environments and humans. By studying and constructing variable impedance actuators and their control, we contribute to the development of actuation units which can match the intrinsic safety, motion performance and energy efficiency of biological systems and in particular the human. As such, this may lead to a new generation of robots that can co-exist and co-operate with people and get closer to the human manipulation and locomotion performance than is possible with current robots.

Variable stiffness actuators realize a novel class of actuators, which are capable of changing the apparent output stiffness independently of the output position. This is mechanically achieved by the internal introduction of a number of elastic elements and a number of actuated degrees of freedom (DOFs), which determine how the elastic elements are sensed at the output. During the nominal behavior of these actuators, the power flow from the internal actuated DOFs can be such that energy is undesirably stored in the elastic elements because of the specific kinematic structure of the actuator. In this study, we focus on the analysis of the power flow in variable stiffness actuators. More specifically, the analysis is restricted to the kinematic structure of the actuators, in order to show the influence of the topological structure on the power flow, rather than on the realization choices. We define a measure that indicates the ratio between the total amount of power that is injected by the internal actuated DOFs and the power that is captured by the internal elastic elements which, therefore, cannot be used to do work on the load. In order to define the power-flow ratio, we exploit a generic port-based model of variable stiffness actuators, which highlights the kinematic properties of the design and the power flows in the actuator structure.

This is a correction to [Duindam and Stramigioli, “Singularity-free dynamic equations of open-chain mechanisms with general holonomic and nonholonomic joints,” IEEE Trans. Robot., vol. 24, no. 3, pp. 527-526, Jun. 2008] where the singularity-free dynamic equations of mechanical systems with Euclidean or non-Euclidean configuration spaces are presented. We present the correct explicit expressions of the equations presented in the above referenced paper.

This video shows the design of a mechanical structure of a miniature pipe inspection robot (MPR) capable of moving trough very small pipes (up to 41 mm inner diameter) as well as a wide range of diameters (63 to 125 mm outer diameter). The requirement to negotiate bends, T-joints and steep inclinations pose another set of strict design constraints. In this video the controlled robot is shown in action.

This paper discusses the design of a mechanical structure of a miniature pipe inspection robot capable of moving through very small pipes (down to 41 mm inner diameter). The requirement to negotiate bends, T-joints and steep inclinations pose another set of strict design constraints. The proposed robot consists of a modular design (7 modules) with a relatively low number of active degrees of freedom. The system is using a novel clamping mechanism with a series-elastic drive. The design of this mechanism has resulted in a high spreading factor allowing the system to operate in a wide diameter range (63 mm to 125 mm outer diameter). In this paper the mechanical design requirements and control system will be discussed. Preliminary test results will be given.

Passivity of bilateral telemanipulation systems ensures stability of the interaction with such systems. In the frequency domain, passivity of a linear time invariant approximation of the system can be designed for a considered set of operating conditions. Non-linear control structures have been proposed that enforce passivity of the system in the time domain. In this paper, extensions are proposed that increase the complimentarity of the frequency- and time domain approaches. The combination of both approaches allows a guaranteed measure of transparency to be designed in the frequency domain for a desired set of operating conditions. For operating conditions outside the desired set, stable interaction is guaranteed by the non-linear passivity enforcing control structure. Simulation results of the combined approach are presented that show that the stability properties of the bilateral controller designed in the frequency domain are improved and the transparency properties are improved with respect to those of the standard passivity-enforcing algorithm in the time domain.

In this paper, we propose a generic model and a controller design for a class of underactuated aerial vehicles, namely for unmanned aerial vehicles whose primary support against gravity is thrust. The approach followed is based on energetic consideration and uses the formalisms of port-Hamiltonian systems and bond graphs. The controller is designed for both stabilization during hovering and for trajectory tracking tasks. The competency of the model and the performance of the controller are validated in simulation.

In this work, we present the design and the implementation of an innovative knee locking mechanism for a dynamically walking robot. The mechanism consists of a four-bar linkage that realizes a mechanical singularity for locking the knee when the leg is in the extended position. Once extended, the knee remains locked without energy consumption, while unlocking it only costs a small amount of energy. Tests showed that the robot walks robustly and that the energy consumption of the new system is low.

In this article we show that the concept of screws and wrenches gives us tools to geometrically establish the relation between the ground reaction wrench and the Zero-Moment Point. In order to arrive at this, we show how a wrench can be decomposed into separate components. The proposed method gives a general, completely coordinate-free way to find the ZMP and contributes in improving the geometrical insight.

The goal of this study is to improve the efficiency and efficacy of the standard colonoscopy procedure. This is done by addressing the intuitiveness of colonoscope control. For this purpose an interface in the form of a grip was designed that allows the user to intuitively steer and drive the colonoscope. The Grip controls the orientation of the tip as if the colonoscope were a stiff instrument that pivots at the anus of a patient. To test the principle, experiments were conducted on a simulator operated by novice subjects. Initial experiments show a significant decrease in introduction time of 156 seconds (p<0.005). This technology will enhance current colonoscopy practice and open up possibilities for future applications of colonoscopy.

A position and orientation sensing system is developed for the feedback control of endoscopic instruments in advanced flexible endoscopes. The images that are taken by the endoscopic camera are used to match a kinematic model to the observed instrument. Using the pseudo-inverse of the Jacobian of the forward kinematics, the estimated state of the model is continuously updated so as to match feature points from the images to the model. An experiment was performed inside a colon model, in which reference markers with known locations were touched with the instrument. The root mean square position estimation errors were 1.7 mm, 1.2 mm and 3.6 mm in the horizontal (x), vertical (y), and away-from-camera (z) directions, respectively.

In this paper, a two-layer approach is presented to guarantee the stable behavior of bilateral telemanipulation systems in the presence of time-varying destabilizing factors such as hard contacts, relaxed user grasps, stiff control settings, and/or communication delays. The approach splits the control architecture into two separate layers. The hierarchical top layer is used to implement a strategy that addresses the desired transparency, and the lower layer ensures that no “virtual” energy is generated. This means that any bilateral controller can be implemented in a passive manner. Separate communication channels connect the layers at the slave and master sides so that information related to exchanged energy is completely separated from information about the desired behavior. Furthermore, the proposed implementation does not depend on any type of assumption about the time delay in the communication channel. By complete separation of the properties of passivity and transparency, each layer can accommodate any number of different implementations that allow for almost independent optimization. Experimental results are presented, which highlight the benefit of the proposed framework.

Variable stiffness actuators are a particular class of actuators that is characterized by the property that the apparent output stiffness can be changed independent of the output position. To achieve this, variable stiffness actuators consist of a number of elastic elements and a number of actuated degrees of freedom, which determine how the elastic elements are perceived at the actuator output. Changing the apparent output stiffness is useful for a broad range of applications, which explains the increasing research interest in this class of actuators. In this paper, a generic, port-based model for variable stiffness actuators is presented, with which a wide variety of designs can be modeled and analyzed. From the analysis of the model, it is possible to derive kinematic properties that variable stiffness actuator designs should satisfy in order to be energy efficient. More specifically, the kinematics should be such that the apparent output stiffness can be varied without changing the potential energy that is stored in the internal elastic elements. A concept design of an energy-efficient variable stiffness actuator is presented and implemented. Simulations of the model and experiments on the realized prototype validate the design principle.

Passivity of virtual environments running in discrete time is a sufficient condition for stability of the system. The framework for passive sampled Port-Hamiltonian systems allows multi-dimensional virtual environments exhibiting internal dynamic behavior to be computed on a discrete medium in a passive manner. It is shown that a causality analysis is required in the framework to detect if any of the model elements have, a time dependent change of energy function in the energy balance of the system. The Standard Linear Solid model, which is often used to simulate the visco-elastic interaction with soft biological tissue is used as an example. Simulated and experimental results are provided to demonstrate the benefit of the described framework. It is shown that using this approach a multi-dimensional model which is passive in the continuous domain remains passive in the discrete domain, whereas a standard discretization approach can become non-passive.

We present a novel, simple and effective approach for tele-operation of aerial robotic vehicles with haptic feedback. Such feedback provides the remote pilot with an intuitive feel of the robot's state and perceived local environment that will ensure simple and safe operation in cluttered 3D environments common in inspection and surveillance tasks. Our approach is based on energetic considerations and uses the concepts of network theory and port-Hamiltonian systems. We provide a general framework for addressing problems such as mapping the limited stroke of a `master' joystick to the infinite stroke of a `slave' vehicle, while preserving passivity of the closed-loop system in the face of potential time delays in communications links and limited sensor data.

In this paper, a metric for comparing different designs of variable stiffness actuators is introduced. For the formulation of this metric, we focus on the energy efficiency of the actuators. In particular, we propose a metric that is a measure of how much energy is used by the actuator for changing the output stiffness. In order to facilitate the analysis of the energy usage, we present a port-based modeling framework, from which design criteria are derived for the optimization of the metric. Finally, the metric is interpreted in a comparison between existing actuators.

In this paper, we provide a port-based mathematical framework for analyzing and modeling variable stiffness actuators. The framework provides important insights in the energy requirements and, therefore, it is an important tool for the design of energy efficient variable stiffness actuators. Based on new insights gained from this approach, a novel conceptual actuator is presented. Simulations show that the apparent output stiffness of this actuator can be dynamically changed in an energy efficient way.

This paper introduces a novel robotic finger concept for variable impedance grasping in unstructured tasks. A brief literature survey reveals the need for minimal component designs and the benefits of impedance control schemes for interaction tasks such as grasping. The novel robotic finger concept supports these insights by combining three key features: minimal actuation, variable mechanical compliance and full manipulability. This combination of features allows for a minimal component design, while reducing control complexity and still providing required dexterity and grasping capabilities. The conceptual properties (such as variable compliance) are studied in a port-Hamiltonian framework. The framework proved to be suitable in analyzing and understanding the finger properties, which will be used for future controller design.

In this paper, we present a port-Hamiltonian model of a multi-fingered robotic hand, with soft-pads, while grasping and manipulating an object. The algebraic constraints of the interconnected systems are represented by a geometric object, called Dirac structure. This provides a powerful way to describe the non-contact to contact transition and contact viscoelasticity, by using the concepts of energy flows and power preserving interconnections. Using the port based model, an Intrinsically Passive Controller (IPC) is used to control the internal forces. Simulation results validate the model and demonstrate the effectiveness of the port-based approach.

In bilateral telemanipulation algorithms based on time-domain passivity, internal friction in the devices poses an additional energy drain. Based on a model of the friction, the dissipated energy can be estimated and reclaimed inside the energy balance of the control algorithm. As long as the estimate is conservative, passivity of the entire system is maintained. In this paper we consider two types of friction and discuss the influence of two types of measurement noise. Without noise compensation the dissipated energy is largely overestimated. A compensation method based on the probability density of the noise is proposed. This leads to an energy estimate which is always conservative even in the presence of measurement noise and does not require additional filtering. Simulation results are provided that show the increase in obtained transparency when this energy compensation technique is applied.

Manually steering the tip of a flexible endoscope to navigate through an endoluminal path relies on the physician's dexterity and experience. In this paper we present the realization of a robotic flexible endoscope steering system that uses the endoscopic images to control the tip orientation towards the direction of the lumen. Two image-based control algorithms are investigated, one is based on the optical flow and the other is based on the image intensity. Both are evaluated using simulations in which the endoscope was steered through the lumen. The RMS distance to the lumen center was less than 25% of the lumen width. An experimental setup was built using a standard flexible endoscope, and the image-based control algorithms were used to actuate the wheels of the endoscope for tip steering. Experiments were conducted in an anatomical model to simulate gastroscopy. The image intensity-based algorithm was capable of steering the endoscope tip through an endoluminal path from the mouth to the duodenum accurately. Compared to manual control, the robotically steered endoscope performed 68% better in terms of keeping the lumen centered in the image.

In this study, we present the conceptual design of a fully-passive transfemoral prosthesis. The design is inspired by the power flow in human gait in order to have an energy efficient device. The working principle of the conceptual mechanism is based on three storage elements, which are responsible of the energetic coupling between the knee and the ankle joints. Design parameters of the prosthesis have been determined according to the energy absorption intervals of the human gait. Simulation results shows that the power flow of the system is comparable with human data. Finally, an initial prototype is presented as proof of concept.

This paper describes the mechatronic design of a humanoid neck. To research human machine interaction, the head and neck combination should be able to approach the human behavior as much as possible. We present a novel humanoid neck concept that is both fast, and has a long range of motion in 4 degrees of freedom (DOFs). This enables the head to track fast objects, and the neck design is suitable for mimicking expressions. The humanoid neck features a differential drive design for the lower 2 DOFs resulting in a low moving mass and the ability to use strong actuators. The performance of the neck has been optimized by minimizing backlash in the mechanisms, and by using gravity compensation. Two cameras in the head are used for scanning and interaction with the environment.

This video shows the results of the project on the mechatronic development of the Twente humanoid head. The mechanical structure consists of a neck with four degrees of freedom (DOFs) and two eyes (a stereo pair system) which tilt on a common axis and rotate sideways freely providing a three more DOFs. The motion control algorithm is designed to receive, as an input, the output of a biological-inspired vision processing algorithm and to exploit the redundancy of the joints for the realization of the movements. The expressions of the humanoid head are implemented by projecting light from the internal part of the translucent plastic cover.

The large number of degrees of freedom in legged robots give rise to complicated dynamics equations. Analyzing these equations or using them for control can therefore be a difficult and non-intuitive task. A simplification of the complex multi-body dynamics can be achieved by instantaneously reducing it to an equivalent single inertial entity called the locked inertia or the composite rigid body inertia.

In this paper the passivity algorithm proposed in is revisited. This algorithm generates passive haptic feedback by monitoring the energy flows in the virtual environment. The original formulation of the algorithm could not deal well with large internal dissipation and high sampling frequencies. An alteration is proposed on how internal dissipation is handled. This alteration improves the robustness of the algorithm for a wider range of parameter values and sample times. The improved version of the algorithm is demonstrated in a virtual wall experiment.

This paper describes the design of a motion control algorithm for a humanoid robotic head, which consists of a neck with four degrees of freedom and two eyes (a stereo pair system) that tilt on a common axis and rotate sideways freely. The kinematic and dynamic properties of the head are analyzed and modeled using screw theory. The motion control algorithm is designed to receive, as an input, the output of a vision processing algorithm and to exploit the redundancy of the system for the realization of the movements. This algorithm is designed to enable the head to focus on and to follow a target, showing human-like motions. The performance of the control algorithm has been tested in a simulated environment and, then, experimentally applied to the real humanoid head.

This paper presents a method to utilize a port-based multibody contact model for simulating dynamic interaction between irregular surfaces. The existing compliant contact model requires an analytic parametrization of the surfaces involved in the interaction, the definition of a Gauss frame in each of the contact points and initialization of the candidate contact points. The authors intend to apply this contact model for cases in which the surfaces of the 3D interacting bodies can not be described (easily) by a geometrically defined surface. Such surfaces are often represented by 3D point meshes. This implies that the surfaces of the bodies have to be reconstructed from 3D point meshes where initialization of the candidate contact points is not arbitrary. This paper proposes the reconstruction of such surfaces by means of polynomial interpolation. With the aim of having a computationally efficient simulator, the surface interpolation is restricted to small patches around the candidate contact points. To select these candidates, the proposed simulation approach is completed by using a fast (existing) collision detection algorithm. Simulation results are validating the effectiveness of the proposed modeling and simulation approach.

In this paper, a systematic procedure for the definition of the dynamical model in port-Hamiltonian form of mechanical systems is presented as the result of the power-conserving interconnection of a set of basic components (rigid bodies, flexible links, and kinematic pairs). Since rigid bodies and flexible links are described within the port-Hamiltonian formalism, their interconnection is possible once a proper relation between the power-conjugated port variables is deduced. These relations are the analogous of the Kirchhoff laws of circuit theory. From the analysis of a set of oriented graphs that describe the topology of the mechanism, an automatic procedure for deriving the dynamical model of a mechanical system is illustrated. The final model is a mixed port-Hamiltonian system, because of the presence of a finite-dimensional subsystem (modeling the rigid bodies) and an infinite-dimensional one (describing the flexible links). Besides facilitating the deduction of the dynamical equations, it is shown how the intrinsic modularity of this approach also simplifies the simulation phase.

This paper deals with the analysis of planar bipedal robots, based on passive dynamic walkers, which are actuated only by actuation of the ankle joints. An overview of the major design characteristics of such robots and their influence on the feasibility of a stable limit cycle is presented. It is shown that robots which are fully powered by ankle actuation require a mass ratio of at least 10:1 between the upper and lower limb to obtain sufficient ground clearance during the swing phase at a wide range of walking speeds. The effect and necessity of the offset in the footshapes of many passive dynamic walkers is shown and the influence of the moment of push off on the required energy injection is treated. The results of the analysis are supported by simulations with a dynamic model of such a robot. The simulated model exhibits a very natural looking gait and walks with a wide range of velocities at low mechanical cost of transport. Simulation results are provided which confirm that pushing off before the swing leg collides with the floor is energetically more efficient than pushing off after the impact as also known from previous literature.

This paper gives a new passivity based control of hydraulic arms based on a new model using ldquonaturalrdquo Casimir functions. Not only passivity but also Casimir functions are used in the modeling and control as a new structural property. First, we refer port-Hamiltonian systems and their properties. Second, we propose two stabilization methods, a new dynamic asymptotic stabilization method and a new partial stabilization method. Third, we give a new model of hydraulic arms using Casimir functions. Furthermore, the proposed two stabilization methods are applied to this model and finally, the validity of our methods are confirmed by not only numerical simulations but also experiments even thought the bulk modulus is not identified at all.

Standard methods to model multibody systems are aimed at systems with configuration spaces isomorphic to Ropfn. This limitation leads to singularities and other artifacts in case the configuration space has a different topology, for example, in the case of ball joints or a free-floating mechanism. This paper discusses an extension of classical methods that allows for a more general class of joints, including all joints with a Lie group structure as well as nonholonomic joints. The model equations are derived using the Boltzmann-Hamel equations and have very similar structure and complexity as obtained using classical methods. However, singularities are avoided through the use of global non-Euclidean configuration coordinates, together with mappings describing a local Euclidean structure around each configuration. The resulting equations are explicit (unconstrained) differential equations, both for holonomic and nonholonomic joints, which do not require a coordinate atlas and can be directly implemented in simulation software.

After stability, transparency is the major issue in the design of a telemanipulation system. In this paper, we exploit the behavioral approach in order to provide an index for the evaluation of transparency in port-Hamiltonian-based teleoperators. Furthermore, we provide a transparency analysis of packet switching scattering-based communication channels.

A new method is presented for controlling the lateral foot placement of a simple 3D compass biped model. The method is based on the fact that, in the limit cycle, the gait is time-reversal symmetric and that, after a disturbance, the degree of asymmetry is indicated by a single variable. This variable is used for feedback with a proportional controller. Simulation results show that the controller works very well for a large range of gaits, without any adaptation of the parameter values

This paper gives new motion generation methods for mechanical port-Hamiltonian systems. First, we propose a generation method based on an asymptotic stabilization method without damping assignment. This asymptotic stabilization method preserves the Hamiltonian structure in the closed-loop system although the controller itself is not a port-Hamiltonian system. Second, we propose another method based on an adaptive asymptotic stabilization method for unknown damping. This adaptive asymptotic stabilizer does not use the value and the sign of the damping at all. Finally, we confirm the effectiveness of our techniques in some numerical simulation.

Standard methods to model multibody systems are aimed at systems with configuration spaces isomorphic to R". This limitation leads to singularities and other artifacts in case the configuration space has a different topology, for example in the case of ball joints or a free-floating mechanism. This paper discusses an extension of classical methods to allow for a very general class of joints, including all joints with a Lie group structure as well as nonholonomic joints. The model equations are derived using the Boltzmann-Hamel equations and have very similar structure and complexity as obtained using classical methods, but they do not suffer from singularities. Furthermore, the equations are explicit differential equations (both for holonomic and nonholonomic joints) and can be directly implemented in simulation software.

Safety is a critical success factor for consumer acceptance of domestic robotic products. Some researchers have adopted the head injury criterion (HIC) as absolute safety norm. However, this norm covers only part of the safety risk. In many cases skin damage (e.g. cuts, wounds, etc) can be a more serious risk. This article shows how to work towards a novel absolute safety measure for evaluating the shape and material choices of a robotic design w.r.t. skin damage. The proposed safety norm evaluates the situation of an unintended uncontrolled collision of a robotic part against a human. Maximum curvatures of the exterior robotic shape are approximated as a sphere in contact with the human skin (locally approximated as a flat surface). This local spheric approximation of the impact contact is used to predict maximum tensile stress during impact of the robotic part on the human. Robotic designs that include points for which the tensile strength of the skin is exceeded will cause at least skin fracture and are therefore considered intrinsically unsafe. While in general applicable, this paper specifically addresses how to apply the proposed norm in the case of safety evaluation of robotic manipulators.

In this paper, a simple way to model flexible robotic links is presented. This is different from classical approaches and from the Euler-Bernoulli or Timoshenko theory, in that the proposed model is able to describe large deflections in 3D space and does not rely on any finite-dimensional approximation (e.g., modal approximation). The model has been formulated within the port Hamiltonian formalism because intuitive considerations on the geometric behavior of the elastic link naturally define a Stokes-Dirac structure, the kernel of a port Hamiltonian system. Moreover, port Hamiltonian systems can be easily interconnected, thus allowing the description of complex systems as a composition of parts in an object-oriented way. By combining rigid bodies, springs, dampers, joints and, finally, flexible links, it is virtually possible to model and mathematically describe whatever complex mechanical structure formed by beams. In order to demonstrate the dynamical properties of the model and how complex mechanisms can be obtained by port interconnection, simulations of 1-DoF and 2-DoF serial manipulators and of a 2-DoF flexible closed kinematic chain are presented.

In this paper, the port Hamiltonian model of a manipulator is presented as the result of the power-conserving interconnection of a set of main components (rigid bodies, flexible links and kinematic pairs). Since rigid bodies and flexible links are described within the port Hamiltonian formalism, their interconnection is possible once a proper relation between the power conjugated port variables is deduced. These relations are the analogous of the Kirchoff laws of circuit theory. The final model is a mixed port Hamiltonian system because of the presence of a finite dimensional subsystem modelling the rigid bodies and of an infinite dimensional one describing the flexible links. The intrinsic modularity of the approach simplifies the model deduction and simulation, while the Hamiltonian description suggests the development of energy-based controllers

In several applications involving haptic interfaces it can be desirable to scale the interaction force perceived by the user. The most intuitive approach is to change the stiffness of the virtual environment but, unfortunately, changing the physical parameters that characterize a virtual environment is a potentially destabilizing action. In this paper we embed in the intrinsically passive haptic scheme recently proposed in S. Stramigioli et al. (2005) a power scaling interconnection that allows to scale the force perceived by the user while preserving the passivity, and consequently the stable behavior, of the overall system

Passivity based bilateral telemanipulation schemes are often subject to a position drift between master and slave if the communication channel is implemented using scattering variables. The magnitude of this position mismatch can be significant during interaction tasks. In this paper we propose a passivity preserving scheme for compensating the position drift arising during contact tasks in port-Hamiltonian based telemanipulation improving the kinematic perception of the remote environment felt by the human operator

The Cayley-Hamilton theorem is an important theorem of linear algebra which is well known and used in system theory. Unfortunately, this powerful result is practically never used in robotics even though it is of extreme relevance. This article is a review of the use of this result for the calculation of general matrix functions which are very common in robotics. It will be shown how any analytic matrix function like exponential, logarithm and more complicated expressions in robotics, can be easily and analytically calculated in an explicit form. Examples are given for the exponential map, inverse of the exponential map, and the derivative of the exponential map. For the first two examples there exist well known expressions in the literature, but the last one is not as easy to compute without the presented methods

After stability, transparency is the major issue in the design of a telemanipulation system. In this paper we exploit a behavioral approach in order to provide an index for the evaluation of transparency in port-Hamiltonian based teleoperators. Furthermore we provide a transparency analysis of packet switching scattering based communication channels.

In several applications involving bilateral telemanipulation, master and slave robots act at different power scales (e.g. telesurgery). The aim of this paper is to embed power scaling into port-Hamiltonian based bilateral telemanipulation schemes, In order to deal with nonnegligible transmission delays we propose a novel scattering based communication strategy to properly scale the power exchanged by master and slave while preserving a stable behavior of the overall scheme.

In this paper, the problem of online estimation of the mechanical impedance during the contact of a robotic system with an unknown environment is considered. This problem is of great interest when controlling a robot in an unstructured and unknown environment, such as in telemanipulation tasks, since it can be easily shown that the exploitation of the knowledge of the mechanical properties of the environment can greatly improve the performance of the robotic system. In particular, a single-point contact is considered, and the (nonlinear) Hunt-Crossley model is taken into account, instead of the classical (linear) Kelvin-Voigt model. Indeed, the former achieves a better physical consistency and also allows describing the behavior of soft materials. Finally, the online estimation algorithm is described and experimental results are presented and discussed.

In this paper, we present a novel way to approach the interconnection of a continuous and a discrete time physical system first presented in . This is done in a way which preserves passivity of the coupled system independently of the sampling time T. This strategy can be used both in the field of telemanipulation, for the implementation of a passive master/slave system on a digital transmission line with varying time delays and possible loss of packets (e.g., the Internet), and in the field of haptics, where the virtual environment should 'feel' like a physical equivalent system.

Research on nonholonomic mechanical systems has focused mainly on describing geometric structure, controllability, and motion planning, yet little attention has been paid to several energy aspects of these systems. This paper describes a method to model nonholonomic mechanical systems as reduced-order port-controlled Hamiltonian systems, in which the energy structure is shown explicitly. We show how very simple equations are obtained for the example of the snakeboard, and then discuss how these equations can be used to derive an energy-based controller in an intuitive way.

In this paper a novel approach to the Cartesian impedance control problem for robots with flexible joints is presented. The proposed controller structure is based on simple physical considerations, which are motivating the extension of classical position feedback by an additional feedback of the joint torques. The torque feedback action can be interpreted as a scaling of the apparent motor inertia. Furthermore the problem of gravity compensation is addressed. Finally, it is shown that the closed loop system can be seen as a feedback interconnection of passive systems. Based on this passivity property a proof of asymptotic stability is presented.

In this paper, the problem of the on-line estimation of the mechanical impedance during the contact of a robotic system with an unknown environment is considered. Indeed, the knowledge of the mechanical properties could allow to improve the interaction between robotic devices and unstructured and unknown environments, e.g. in telemanipulation tasks. A single-point contact is considered and the (nonlinear) Hunt-Crossley model is taken into account and its better physical consistency in describing the behavior of soft materials is discussed in comparison with the classical (linear) Kelvin-Voigt model. Finally, the on-line estimation algorithm is described and experimental results presented.

In this paper, we derive a coordinate-free formulation of a passive controller that makes a mechanical system track reference curves in a potential field. Contrary to conventional reference tracking, we do not specify a single time-varying trajectory that the system has to track. Instead, we specify a whole curve that the system has to stay on at all times. Using tools from differential geometry, we first derive a controller that makes the system move along arbitrary (smooth enough) reference curves while keeping the kinetic energy constant. We then apply the results to the case of movement in an artificial potential field, in which case, the reference curves are completely determined by the potential field and cannot be chosen arbitrarily. Simulation then shows the performance of the controller on a benchmark robot with two degrees of freedom.

In this paper, we discuss the modeling of the kinematics and dynamics of compliant contact between bodies moving in Euclidean space. First, we derive the kinematic equations describing the motion of the contact point when two rigid bodies are rolling on each other. Secondly, we extend these results to describe the motion of the closest points between two rigid bodies moving freely in space. Then, we use these results to model compliant contact between bodies, using a spatial spring and a damper to model energy stored and dissipated during contact.

In this paper we present an intrinsically passive telemanipulation scheme over a digital transmission line Internet-like. We present an analysis of the energetic behavior of the communication line both in case of loss of packages and in case of variable delay. The sample data nature of the passive controller is explicitly taken into account following the approach outlined.

In this paper two problems arising in the digital passive scheme for telemanipulation presented in are addressed. At first, we show how to preserve system passivity in presence of quantization error introduced by position sensor (i.e. encoders) by introducing energy dissipation. Then, we introduce a scheme for a redundant communication channel that will compensate for missed packets improving performances while preserving passivity of the overall scheme.

In this paper the problem of delayed virtual environments in haptics is addressed. We show that the approach outlined is no longer passive in case of (computational) delay on the output of the virtual environment. Passivity can be recovered using scattering theory; a discretization algorithm which leads to a discrete passive port-Hamiltonian systems with respect to any delay on the output is proposed.

This paper presents a novel approach to the interconnection of a continuous time and a discrete time physical system. This is done in a way which preserves the passivity of the coupled system independently of the sampling time. A direct application in the field of haptic displays, where a virtual environment should feel like equivalent physical systems, is presented.

This paper presents an elegant contact dynamics model in screw bondgraph form. It can model the contact between any two objects of finite curvature. It does so by defining a Gauss frame on the surfaces of both objects in the points that are closest to each other. Then it describes how the Gauss frames move as the objects move relative to each other. This is called the contact kinematics. The contact kinematics detect when two objects touch, then describe how they roll and slide over and along each other, and then detect when they get loose again. With the contact kinematics a dynamic screw bondgraph model is built. This dynamics model is verified in two simulations. The first simulation shows two objects of nontrivial curvature, eggs, rolling over each other. The second shows a robotic hand manipulating an object.

In this paper, we study the interconnection of two robots, which are modeled as port-controlled Hamiltonian systems through a transmission line with time delay. There will be no analysis of the time delay, but its presence justifies the use of scattering variables to preserve passivity. The contributions of the paper are twofold: first, a geometrical, multidimensional, power-consistent exposition of telemanipulation of intrinsically passive controlled physical systems, with a clarification on impedance matching, and second, a system theoretic condition for the adaptation of a general port-controlled Hamiltonian system with dissipation (port-Hamiltonian system) to a transmission line.

This article shows that time derivatives of twists and wrenches are indeed screws, in contrast to many classical kinematicians' believe. Furthermore, it is proven that the "centripetal screw" as well as the momentum of a rigid body together with all its derivatives, are also screws, and that a rigid body's dynamics can be geometrically expressed as a screw equation. The paper relies on a somewhat more formal treatment of the screw theory than usual, in order to clarify these "controversial" issues concerning the motion of rigid systems, and in order to make the link with the more general (and historically much richer) field of differential geometry.

In this paper, an extension of the so-called intrinsic passive control (IPC) is illustrated, showing that an improvement of performances can be achieved by considering different types of energy-storing elements, i.e. "springs", in the IPC. In particular, two new "springs" are introduced: a 'variable rest length' spring and a 'variable stiffness' spring, that are properly defined in order to maintain the passivity of the IPC and to improve its performances in given situations. Simulations of the resulting control, applied to a defective system and to a simple telemanipulation device, are presented and discussed.

This article presents a passive way to implement varying spatial springs. These are springs with controlling ports which can be used to modify their spatial rest length or spatial properties. These controlling ports have a dual structure which allows one to supervise the potential energy injected into the spring by varying its properties. A direct application in tele-manipulation using geometric scattering is briefly described.

Shows that in the use of Lie groups for the study of the relative motion of rigid bodies some assumptions are not explicitly stated. A commutation diagram is shown which points out the "reference problem" and its simplification to the usual Lie group approach under certain conditions which are made explicit.

This article presents a robotics architecture for impedance controlled manipulators. The impedance superposition principle is exploited in order to build a merging mechanism for behaviour-based architectures. A preliminary set of experiments shows the potentiality of the proposed approach.